1. Field of the Invention
The present invention relates to a relative velocity control device for use with a magnetic recording and reproducing apparatus of the rotating head type, wherein head rotating and tape running speeds are controlled to keep the relative velocity between head and tape constant at the time when the tape is running at high speed.
2. Technical Background of the Invention
The recording density of the tape which is used in the magnetic recording and reproducing apparatus of the rotating head type has, in recent times, been enhanced and a plurality of programs are often recorded on the same tape. It is thus needed to search and tetrieve the programs. In the case of the rotating head type VTR, the manner in which an exclusive track is provided on the tape to retrieve the recorded programs and recording address signals on the track in order to retrieve and edit the programs has been widely employed by businesses. In the case of the home VTR, however, the tape is narrow and it is therefore not desirable from the viewpoint of the use efficiency of the tape that the track exclusively used to retrieve the programs is provided on the tape. In addition, a fixed head for the track must be added, thereby making the cost high. Therefore, the address signals are recorded on the video track together with main signals. In this case, the address signal which has been digitized is recorded at a predetermined position on every video track.
In the case of reproducing digital signals which have been recorded at high density, the rising and falling of reproduced waveform are generally inclined and its peak position is shifted because of intersymbol interference. Therefore, a bit clock is generated by the PLL circuit after waveform equalization and the digital signal is reproduced and demodulated by this bit clock. In this case, when the relative velocity between the tape and the head (which will be hereinafter referred to as the relative velocity) is changed, the frequency characteristic of the waveform equalizer circuit and capture and lock ranges of the PLL circuit must be changed according to the change of the relative velocity. The relative velocity therefore needs to be substantially constant. For the purpose of retrieving the programs at high speed, it is necessary that the address signals recorded on the tape are read while the tape is being run at a speed higher than that at the recording time. When the tape is run at a higher speed than that at the recording time, however, the relative velocity becomes different from that at the recording time and the address signals cannot be read stably, provided that the number of drum rotation and the angle of the drum inclined relative to the tape is constant. For the purpose of reading the address signals, therefore, the relative velocity also needs to be constant. Program retrieving is carried out, running the tape at high speed in a forward or reverse direction; the ratio of the relative velocities in the forward and reverse directions increases several times.
When no track exclusively used to retrieve the programs is provided, there is also no exclusive track for the speed detection which is needed to make the tape running speed constant. When a track exclusively used to retrieve the programs is provided, the fixed head for the track is added. Therefore, a signal which represents the tape running speed can be obtained from this head and the tape running speed can be kept by this signal. When no track exclusively used to retrieve the programs is provided, however, the speed detection must be carried out by another added means.